A variety of compositions and processes have heretofore been used or proposed for use for effecting the formation of an adherent, uniform phosphate coating on metal surfaces to enhance the adhesion of subsequently applied siccative coatings and to provide for substantially improved corrosion resistance of the coated substrate. Phosphate coating processes of the various types well known and practiced in the art conventionally comprise aqueous acidic solutions which are applied to a cleaned metal surface such as by spraying, immersion, flooding or the like for a period of time sufficient to form a crystalline phosphate coating thereon. Numerous improvements in such phosphate coating processes and compositions have been made since their early introduction including the development of an activating pretreatment to accelerate the formation of a phosphate coating on a metal surface and to increase the uniformity of the coating produced. One such activating treatment is taught in U.S. Pat. No. 2,310,239 to Jernstedt in which the cleaned metal surface is first contacted with an aqueous dispersion of a dried disodium phosphate-titanium compound reaction product containing from about 0.005 to about 20 percent by weight titanium prior to contact with the subsequent phosphating solution. An improved method for producing the phosphate-titanium reaction product is taught in U.S. Pat. No. 2,874,081 to Cavanagh and Maurer the teachings of which are incorporated herein by reference.
The continuing demand for still further improvements in the corrosion resistance of metal substrates, particularly ferrous-base, zinc-base and aluminum-base substrates as employed in automotive applications has provided impetus in the development of still further improvements to substantially enhance corrosion protection and adherence of various siccative organic finishes including electrophoretic coatings applied to automobile body and chassis components. The advent of more stringent corrosion test procedures adopted by automobile manufacturers including a so-called cycle test as an indication of the projected service life of a treated automotive component has rendered many of the prior art phosphating processes and compositions deficient in passing such rigorous test conditions.
The present invention provides for an improved activation composition and process for activating metal surfaces prior to a phosphating treatment employing comparatively low concentrations of the activating constituents thereby achieving substantial economy in the treatment of metal surfaces while at the same time producing comparatively thin, dense, fine-size crystalline adherent phosphate coatings providing superior corrosion protection in comparison to prior art compositions and processes heretofore known.